Description:
------------
The calendar extension's unixtojd() function is described in the php manual as simply:

   unixtojd ? Convert Unix timestamp to Julian Day

No further explanation is given for how a Julian Day is defined in this case.  However, if we use the definition given at <http://www.decimaltime.hynes.net/dates.html#jdn> which appears to be the normal astronomical meaning of a Julian Day, a Julian Day should be measured from noon GMT (or UT).

The problem is that the unixtojd() function uses php_localtime_r() to convert a given UNIX timestamp to a time_t struct - which means the base Gregorian date used for the conversion is calculated in the local timezone, not in GMT.  This function should use php_gmtime_r() instead.

This problem was detected when running the PHP testsuite with TZ=UTC set - the ext/calendar/tests/unixtojd.phpt test expects a value that is only returned in a timezone west of GMT!
(Separately, I see nowhere that the 12h offset is taken into account, so in practice all unixtojd() results are currently calculated from midnight in the local timezone.)

Reproduce code:
---------------
<?php
setenv('TZ=America/Los_Angeles');
echo unixtojd(1000000000). "\n";
?>


Expected result:
----------------
2452162

Actual result:
--------------
2452161